The invention relates generally to the rapid, high resolution inspection of circuit boards using a computerized laminography system, and in particular, to systems which use electronic means to adjust the Z-axis location of the inspection site with respect to the circuit board.
Rapid and precise quality control inspections of the soldering and assembly of electronic devices have become priority items in the electronics manufacturing industry. The reduced size of components and solder connections, the resulting increased density of components on circuit boards and the advent of surface mount technology (SMT), which places solder connections underneath device packages where they are hidden from view, have made rapid and precise inspections of electronic devices and the electrical connections between devices very difficult to perform in a manufacturing environment.
Many existing inspection systems for electronic devices and connections make use of penetrating radiation to form images which exhibit features representative of the internal structure of the devices and connections. These systems often utilize conventional radiographic techniques wherein the penetrating radiation comprises X-rays. Medical X-ray pictures of various parts of the human body, e.g., the chest, arms, legs, spine, etc., are perhaps the most familiar examples of conventional radiographic images. The images or pictures formed represent the X-ray shadow cast by an object being inspected when it is illuminated by a beam of X-rays. The X-ray shadow is detected and recorded by an X-ray sensitive material such as film or other suitable means.
The appearance of the X-ray shadow or radiograph is determined not only by the internal structural characteristics of the object, but also by the direction from which the incident X-rays strike the object. Therefore, a complete interpretation and analysis of X-ray shadow images, whether performed visually by a person or numerically by a computer, often requires that certain assumptions be made regarding the characteristics of the object and its orientation with respect to the X-ray beam. For example, it is often necessary to make specific assumptions regarding the shape, internal structure, etc. of the object and the direction of the incident X-rays upon the object. Based on these assumptions, features of the X-ray image may be analyzed to determine the location, size, shape, etc., of the corresponding structural characteristic of the object, e.g., a defect in a solder connection, which produced the image feature. These assumptions often create ambiguities which degrade the reliability of the interpretation of the images and the decisions based upon the analysis of the X-ray shadow images. One of the primary ambiguities resulting from the use of such assumptions in the analysis of conventional radiographs is that small variations of a structural characteristic within an object, such as the shape, density and size of a defect within a solder connection, are often masked by the overshadowing mass of the solder connection itself as well as by neighboring solder connections, electronic devices, circuit boards and other objects. Since the overshadowing mass and neighboring objects are usually different for each solder joint, it is extremely cumbersome and often nearly impossible to make enough assumptions to precisely determine shapes, sizes and locations of solder defects within individual solder joints.
In an attempt to compensate for these shortcomings, some systems incorporate the capability of viewing the object from a plurality of angles. One such system is described in U.S. Pat. No. 4,809,308 entitled xe2x80x9cMETHOD and APPARATUS FOR PERFORMING AUTOMATED CIRCUIT BOARD SOLDER QUALITY INSPECTIONSxe2x80x9d, issued to Adams et al. The additional views enable these systems to partially resolve the ambiguities present in the X-ray shadow projection images. However, utilization of multiple viewing angles necessitates a complicated mechanical handling system, often requiring as many as five independent, non-orthogonal axes of motion. This degree of mechanical complication leads to increased expense, increased size and weight, longer inspection times, reduced throughput, impaired positioning precision due to the mechanical complications, and calibration and computer control complications due to the non-orthogonality of the axes of motion.
Many of the problems associated with the conventional radiography techniques discussed above may be alleviated by producing cross-sectional images of the object being inspected. Tomographic techniques such as laminography and computed tomography (CT) have been used in medical applications to produce cross-sectional or body section images. In medical applications, these techniques have met with widespread success, largely because relatively low resolution on the order of one or two millimeters (0.04 to 0.08 inches) is satisfactory and because speed and throughput requirements are not as severe as the corresponding industrial requirements.
In the case of electronics inspection, and more particularly, for inspection of electrical connections such as solder joints, image resolution on the order of several micrometers, for example, 20 micrometers (0.0008 inches) is preferred. Furthermore, an industrial solder joint inspection system must generate multiple images per second in order to be practical for use on an industrial production line. Laminography systems which are capable of achieving the speed and accuracy requirements necessary for electronics inspection are described in the following patents: 1) U.S. Pat. No. 4,926,452 entitled xe2x80x9cAUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICSxe2x80x9d, issued to Baker et al.; 2) U.S. Pat. No. 5,097,492 entitled xe2x80x9cAUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICSxe2x80x9d, issued to Baker et al.; 3) U.S. Pat. No. 5,081,656 entitled xe2x80x9cAUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICSxe2x80x9d, issued to Baker et al.; 4) U.S. Pat. No. 5,291,535 entitled xe2x80x9cMETHOD AND APPARATUS FOR DETECTING EXCESS/INSUFFICIENT SOLDER DEFECTSxe2x80x9d, issued to Baker et al.; 5) U.S. Pat. No. 5,621,811 entitled xe2x80x9cLEARNING METHOD AND APPARATUS FOR DETECTING AND CONTROLLING SOLDER DEFECTSxe2x80x9d, issued to Roder et al.; 6) U.S. Pat. No. 5,561,696 xe2x80x9cMETHOD and APPARATUS FOR INSPECTING ELECTRICAL CONNECTIONSxe2x80x9d, issued to Adams et al.; 7) U.S. Pat. No. 5,199,054 entitled xe2x80x9cMETHOD AND APPARATUS FOR HIGH RESOLUTION INSPECTION OF ELECTRONIC ITEMSxe2x80x9d, issued to Adams et al.; 8) U.S. Pat. No. 5,259,012 entitled xe2x80x9cLAMINOGRAPHY SYSTEM AND METHOD WITH ELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCExe2x80x9d, issued to Baker et al.; 9) U.S. Pat. No. 5,583,904 entitled xe2x80x9cCONTINUOUS LINEAR SCAN LAMINOGRAPHY SYSTEM AND METHODxe2x80x9d, issued to Adams; and 10) U.S. Pat. No. 5,687,209 entitled xe2x80x9cAUTOMATIC WARP COMPENSATION FOR LAMINOGRAPHIC CIRCUIT BOARD INSPECTIONxe2x80x9d, issued to Adams. The entirety of each of the above referenced patents is hereby incorporated herein by reference.
Several of the above referenced patents disclose devices and methods for the generation of cross-sectional images of test objects at a fixed or selectable cross-sectional image focal plane in these systems, an X-ray source system and an X-Ray detector system are separated in the Z-axis direction by a fixed distance and the cross-sectional image focal plane is located at a predetermined specific position on the Z-axis which is intermediate the Z-axis locations of the X-ray source system and the X-ray detector system. The X-Ray detector system collects data from which a cross-sectional image of features in the test object, located at the cross-sectional image focal plane, can be formed. In systems having a fixed cross-sectional image focal plane, it is necessary to postulate that the features desired to be imaged are located in the fixed cross-sectional image focal plane at the predetermined specific position along the Z-axis. Thus, in these systems, it is essential that the positions of the fixed cross-sectional image focal plane and the plane with respect to the object which is desired to be imaged, be configured to coincide at the same position along the Z-axis. If this condition is not met, then the desired image of the selected feature of the test object will not be acquired. Instead, a cross-sectional image of a plane with respect to the test object which is either above or below the plane which includes the selected feature will be acquired. Thus, mechanical motion of the test object along the Z-axis is often used to position the desired plane with respect to the test object which is to be imaged at the position of the fixed cross-sectional image focal plane of the inspection system.
Since the laminographic image area (e.g., 2-3 cm2) of a typical laminography system is substantially smaller than the area of a typical circuit board (e.g., 150-1,500 cm2), a complete inspection of a circuit board includes multiple laminographic images, which, if pieced together would form an image of the entire circuit board or selected regions of the circuit board. Thus, in addition to having a Z-axis mechanical positioning system for placing the test object (circuit board) at a specific location along the Z-axis, a typical high resolution laminography system also includes X-axis and Y-axis mechanical positioning systems for placing the test object at specific locations along the X and Y axes. This is frequently achieved by supporting the test object on a mechanical handling system, such as an X,Y,Z positioning table. The table is then moved to bring the desired regions of the test object into the laminographic image area of the laminography system. Movement in the X and Y directions locates the region of the test object to be examined, while movement in the Z direction moves the test object up and down to select the plane with respect to the test object where the cross sectional image is to be taken. As used throughout this document, the phrase xe2x80x9cboard viewxe2x80x9d will be used to refer to the laminographic image of a particular region or area of a circuit board identified by a specific X,Y coordinate of the circuit board. Thus, each xe2x80x9cboard viewxe2x80x9d includes only a portion of the circuit board.
Many inspections require that some of the board views include multiple images at different Z-axis levels of the circuit board. This may be accomplished by physically moving the circuit board up or down in the Z-axis direction using the X,Y,Z mechanical positioning table. However, this additional mechanical motion along the Z-axis direction can also lead to increased expense, increased size and weight, longer inspection times, reduced throughput, reduced image resolution and accuracy due to mechanical vibrations and impaired Z-axis positioning precision due to mechanical complications.
An alternative to mechanical Z-axis positioning is disclosed in U.S. Pat. No. 5,259,012 entitled xe2x80x9cLAMINOGRAPHY SYSTEM AND METHOD WITH ELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCExe2x80x9d, issued to Baker et al. This patent describes a laminography system which electronically shifts the Z-axis location of the image plane with respect to the test object. In this device, the test object is interposed between a rotating X-Ray source and a synchronized rotating X-ray detector. A focal plane with respect to the test object is imaged onto the detector so that a cross-sectional image of a layer of the test object which coincides with the image focal plane is produced. The X-ray source is produced by deflecting an electron beam onto a target anode. The target anode emits X-ray radiation where the electrons are incident upon the target anode. The electron beam is produced by an electron gun which includes X and Y deflection coils for deflecting the electron beam in the X and Y directions. The X and Y deflection coils cause the X-ray source to rotate in a circular trace path. The voltages applied to the X and Y deflection coils are adjusted to change the radius of the circular trace path on the target anode resulting in a change in the Z-axis location of the image plane with respect to the test object. A characteristic of this type of electronic Z-axis positioning system is that images produced at different Z-axis positions have different magnification factors. The different magnification factors of the images complicates the analysis of the multiple images acquired during a complete inspection of the circuit board.
In summary, the magnification of multiple board views at different Z levels is not changed when using systems of the type previously described wherein the X-ray source and detector are fixed at specific locations along the Z-axis and the circuit board is moved in the Z-axis direction to obtain laminographic images at the different Z levels of the circuit board. Alternatively, the magnification of different Z level board views does change with each change in Z level when using the previously described systems which electronically change the radius of the X-Ray source to obtain laminographic images at different Z levels of the circuit board. The different magnifications for different Z level board views in these systems presents difficulties in analyzing the images thus obtained.
The present invention provides improvements which address the above listed specific problems. The present invention advantageously includes ease of use and improved accuracy of Z elevation determination, resulting in an improved technique for producing high resolution cross sectional images of electrical connections.
The present invention comprises an improved computerized laminography system which accurately compensates for variable magnifications of different Z level board views in an efficient manner. This feature makes it feasible to eliminate the Z-axis mechanical motion of the circuit board along the Z-axis direction. Elimination of the Z-axis mechanical motion improves speed of the inspection as well as reliability of the inspection system.
As used throughout this document, the phrase xe2x80x9cfield of viewxe2x80x9d or xe2x80x9cFOVxe2x80x9d will be used to refer to the size of a particular region or area of a circuit board which is included in a laminographic image of that particular region or area of the circuit board. For example, one particular configuration of the present invention has two preset magnification factors. A first magnification factor of 4.75 has a FOV of 0.8 inchxc3x970.8 inch and an image size of 3.8 inchesxc3x973.8 inches. Thus, a board view at a particular x,y location of the circuit board at a magnification of 4.75 refers to a 3.8 inchesxc3x973.8 inches image of a 0.8 inchxc3x970.8 inch region of the circuit board centered at location x,y on the circuit board. A second magnification factor of 19 has a FOV of 0.2 inchxc3x970.2 inch and an image size of 3.8 inchesxc3x973.8 inches. Thus, a board view at a particular x,y location of the circuit board at a magnification of 19 refers to a 3.8 inchesxc3x973.8 inches image of a 0.2 inchxc3x970.2 inch region of the circuit board centered at location x,y on the circuit board. Thus, four board views at the magnification of 19, each board view having a FOV of 0.2 inchxc3x970.2 inch, are required to image the single corresponding board view at the magnification of 4.75, each board view having a FOV of 0.8 inchxc3x970.8 inch. In terms of FOV, the FOV of the system operating at a magnification factor of 4.75 is 4 times larger than the FOV of the system operating at a magnification factor of 19.
As described above, in addition to changing the magnification of the image, another side effect of changing the Z-axis location of the image plane electronically as opposed to mechanically is that the field of view (FOV) for different Z-axis locations of the image plane also changes as the magnification changes. In systems having a fixed Z-axis location of the image plane, the magnification and FOV are not dependent on which Z-level of the circuit board is being imaged since different Z-levels of the circuit board are mechanically positioned at the same fixed Z-axis location of the image plane of the system.
There are several ways that this change in FOV with magnification can be accounted for and corrected in analyzing the images. In circuit board inspection systems, CAD data which describes the circuit board being inspected is utilized during the acquisition and analysis of the images of the circuit board. Thus, a first technique for compensating for variable image magnification factors and FOV""s may be accomplished by magnifying or shrinking the acquired images to a xe2x80x9cnominalxe2x80x9d size (xe2x80x9cnominalxe2x80x9d being defined by a base FOV). Numerous algorithms for doing this are well documented in the technical literature. However, these techniques tend to be CPU intensive and may affect throughput of the system. A second and preferred technique for compensating for variable image magnification factors and FOV""s may be accomplished more efficiently by using on-the-fly CAD data manipulation and on-the-fly FOV adjustments during the analysis of the images.
In a first aspect, the present invention is a device for inspecting electrical connections on a circuit board comprising: a source of X-rays which emits X-rays through the electrical connection from a plurality of positions centered about a first radius and a second radius; an X-ray detector system positioned to receive the X-rays produced by the source of X-rays which have penetrated the electrical connection, the X-ray detector system further comprising an output which emits data signals; an image memory which combines the detector data signals to form an image database which contains information sufficient to form a first cross-sectional image of a cutting plane of the electrical connection at a first image plane at a first Z-axis location corresponding to the first X-ray source radius and a second cross-sectional image of a cutting plane of the electrical connection at a second image plane at a second Z-axis location corresponding to the second X-ray source radius; and a processor which controls the acquisition and formation of the cross-sectional images and analyzes the cross-sectional images, the image processor further comprising: a storage area for storing CAD data which describes a first cross-sectional design of the electrical connection at the first image plane at the first Z-axis location and CAD data for a second cross-sectional design of the electrical connection at the second image plane at the second Z-axis location; and a CAD data calculator section which determines a variance between the first cross-sectional image at the first image plane and the second cross-sectional image at the second image plane and uses the variance to modify, on an as-needed basis, portions of the CAD data which describe said electrical connection at the second image plane at the second Z-axis location thereby generating modified CAD data for the second image plane which describes the electrical connection at the second image plane as represented by the second cross-sectional image. In some configurations, the first cross sectional image has a first field of view and the second cross sectional image has a second field of view and the variance between the first cross-sectional image and the second cross-sectional image is determined by comparing the second field of view to the first field of view. In some configurations, the first cross sectional image has a first magnification factor and the second cross sectional image has a second magnification factor and the variance between the first cross-sectional image and the second cross-sectional image is determined by comparing the second magnification factor to the first magnification factor. In some configurations, the source of X-rays comprises a plurality of X-ray sources. In some configurations, the X-ray detector system comprises a plurality of X-ray detectors. In some configurations, the processor further comprises an image section which produces the cross-sectional images of the electrical connection from the image database.
A second aspect of the present invention includes a method for analyzing laminographic images of an object at multiple Z-axis levels within the object comprising the steps of: determining a reference Z-axis position Z1 corresponding to a first Z level in the object; acquiring a first cross sectional image of the object at the reference Z-axis position Z1 which corresponds to the first Z level in the object and a second cross sectional image of the object at a second Z-axis position Z2 which corresponds to a second Z level in the object; providing first Z level design data which describes the object and specific features within the object at the first Z level of the object and second Z level design data which describes the object and specific features within the object at the second Z level of the object; determining a variance factor which represents a difference between the first cross sectional image of the object at the first Z level and the second cross sectional image of the object at the second Z level; and modifying in real time or near real time, one or more portions of the second Z level design data with the variance factor while comparing the second cross sectional image of the object at the second Z level with the real time or near real time modified second Z level design data. In some implementations of the method, the first cross sectional image has a first field of view and the second cross sectional image has a second field of view and the variance factor which represents a difference between the first cross-sectional image and the second cross-sectional image is determined by comparing the second field of view to the first field of view. In some implementations of the method, the first cross sectional image has a first magnification factor and the second cross sectional image has a second magnification factor and the variance factor which represents a difference between the first cross-sectional image and the second cross-sectional image is determined by comparing the second magnification factor to the first magnification factor.
A third aspect of the present invention includes a method for inspecting an electrical connection on a circuit board comprising: determining a first Z-axis position Z1 corresponding to a first Z level in the electrical connection; acquiring a first cross sectional image of the electrical connection at the first Z-axis position Z1 which corresponds to the first Z level the electrical connection and a second cross sectional image of the electrical connection at a second Z-axis position Z2 which corresponds to a second Z level in the electrical connection, wherein the first cross sectional image has a first magnification factor and the second cross sectional image has a second magnification factor; providing first Z level design data which describes the electrical connection and specific design features within the electrical connection at the first Z level of the electrical connection and second Z level design data which describes the electrical connection and specific design features within the electrical connection at the second Z level of the electrical connection; comparing the first and second magnification factors to determine a first field of view correction factor; and modifying in real time or near real time, one or more portions of the second Z level design data with the first field of view correction factor while comparing the second cross sectional image of the electrical connection at the second Z level with the real time or near real time modified second Z level design data. Some implementations of this method further comprise: providing third Z level design data which describes the electrical connection and specific design features within the electrical connection at a third Z level of the electrical connection; acquiring a third cross sectional image of the electrical connection at a third Z-axis position Z3 which corresponds to the third Z level in the electrical connection wherein the third cross sectional image has a third magnification factor; comparing the first and third magnification factors to determine a second field of view correction factor; and modifying in real time or near real time, one or more portions of the third Z level design data with the second field of view correction factor while comparing the third cross sectional image of the electrical connection at the third Z level with the real time or near real time modified third Z level design data.
These and other characteristics of the present invention will become apparent through reference to the following detailed description of the preferred embodiments and accompanying drawings.